An application of such sensors is in particular radiological imaging, either direct (photosensitive matrix sensors covered with a scintillator), or indirect (matrix sensor at the output of a radiological image intensifier tube). Typically, in the case of a sensor at the output of an intensifier tube, to get real-time images (30 images per second), with a very considerable dynamic range (up to at least 150,000 electrons per image point) and a high resolution (at least 1000×1000 points), it turns out to be necessary to divide the sensitive surface into two or four (or even more) image zones. The matrix sensor has as many outputs as there are zones. These outputs supply for example signals at a rate of 20 MHz if there were just one output, it would be necessary to multiply the output rate by two, or four, or more, but this is difficult if the dynamic range is considerable, the transfer time for large quantities of electrons being high.
The electronic signals arising from the various zones travel via different pathways until they join up into a stream of digital data representing the entire image. These pathways are theoretically identical but may have slight differences (different sensitivities, different amplifying coefficients, different offset voltages, etc). As a result, a uniform global image will clearly show up visible transitions between zones whereas these transitions do not exist in the original image. These transitions are visible on a uniformly grey image: one sees a chessboard where one ought only to see a uniform surface. However, they are also visible on any image, the eye being especially sensitive to correlated transitions in level. Here, the transitions are correlated in the sense that although the global image is any image whatsoever, the changes, even very slight (less than 1% difference in level), in grey level are situated on border lines, which are in general straight. The eye immediately spots these abnormal transitions in an image which a priori has no reason to contain them.
It has already been proposed that this phenomenon be corrected by passing the signal arising from the sensor, in analog form, to nonlinear correction circuits, correcting first-order nonlinearities of the various pathways. This solution is expensive in terms of circuitry, and rather inaccurate. It cannot correct nonlinearities of complex forms.
Another solution could be, in the case of an analog/digital conversion of the signal arising from the sensor and a subsequent digital processing, to consider a first processing pathway as reference pathway; the other pathways would be pathways to be corrected and would each comprise a digital correction table whose content would compensate for the differences between this pathway and the reference pathway. The signals of the reference pathway are transmitted as they are, and the signals of the pathways to be corrected are modified by the respective correction tables before being amalgamated with the signals of the reference pathway so as to reconstruct a global image amalgamating the various zones.
The correction table, or “look-up table”, is a memory which matches a corrected value with each digital value representing a signal level. All the sensitivity differences, the nonlinearities, or the shifts in level, between the various pathways, can thus be very finely corrected.
The construction of the content of the various correction tables will thus in principle be done in the factory, on the basis of grey scale charts making it possible to detect, for each grey level, which behavior is exhibited by each signal processing pathway, so that for any grey level whatsoever, the final digital value output is the same whatever the processing pathway.
The procedure for constructing these correction tables, which differ from one image zone to another, and which differ from one product to another since not all the parameters which engender the small differences between pathways are controlled during production, is unwieldy and expensive. Furthermore, it cannot, without severely increasing the maintenance costs, be repeated periodically to take account of the aging of the products or of modifications due to ambient conditions (temperature in particular).
It is also possible to envisage the use of correction tables which, instead of being established on the basis of frozen images (grey scale charts), are established directly, in dynamic mode, in tandem with the use of the image sensor, on the basis of the images actually observed by the sensor. The table is not therefore contained in a read-only memory since its content is continually reappraised through the use of the sensor. The observation of image discontinuities for a luminance level present at the border between the two half-images, leads to the introduction into the table, for this luminance level value, of a correction coefficient aimed at eliminating this discontinuity.
However, the solutions indicated hereinabove are not entirely satisfactory from the point of view of correction performance, or they require overly complex correction algorithms. The purpose of the present invention is therefore to propose a device for image capture which does not exhibit the drawbacks of the prior art devices or of the devices using correction tables such as described hereinabove, and which efficiently correct the differences in the processing pathways for the various zones of an image.